Respiratory syncytial virus (RSV) is an enveloped non-segmented negative-strand RNA virus in the family Paramyxoviridae, genus Pneumovirus. Worldwide, it is estimated that 64 million RSV infections occur each year resulting in 160,000 deaths (WHO Acute Respiratory Infections Update September 2009). The most severe disease occurs particularly in premature infants, the elderly and immune-compromised individuals. In children younger than 2 years, RSV is the most common respiratory tract pathogen, accounting for approximately 50% of the hospitalizations due to respiratory infections, with a peak of hospitalization occurring at 2-4 months of age. It has been reported that almost all children have been infected by RSV by the age of two. Repeated infection during lifetime is attributed to ineffective natural immunity. The level of RSV disease burden, mortality and morbidity in the elderly are second only to those caused by non-pandemic influenza A infections.
To infect a host cell, RSV, like other enveloped viruses such as influenza virus and HIV, require fusion of the viral membrane with a host cell membrane. For RSV, the conserved fusion protein (RSV F protein) fuses the viral and host cell cellular membranes. In current models, based on paramyxovirus studies, the RSV F protein initially folds into a “pre-fusion” conformation. The metastable structure has recently been solved in complex with a stabilizing neutralizing antibody Fab fragment (McLellan et al., Science 340(6136):1113-7, 2013). During cell entry, the pre-fusion conformation undergoes refolding and conformational changes to its “post-fusion” conformation (McLellan, J. Vivol. 85(15):7788-96, 2010; Swanson, PNAS 108(23):9619-24, 2011). Thus, the RSV F protein is a metastable protein that drives membrane fusion by coupling irreversible protein refolding to membrane juxtaposition by initially folding into a metastable form (pre-fusion conformation) that subsequently undergoes discrete/stepwise conformational changes to a lower energy conformation (post-fusion conformation).
These observations suggest that pre-fusion and post-fusion RSV F protein are antigenically distinct (L. J. Calder et al., Virology 271:122-131 (2000)).
A vaccine against RSV infection is not currently available, but is desired. Vaccine candidates based on the RSV F protein have failed due to problems with, e.g., stability, purity, reproducibility, and potency. As indicated above, crystal structures have revealed a large conformational change between the pre-fusion and post-fusion states. The magnitude of the rearrangement suggested that only a portion of antibodies directed to the post-fusion conformation of RSV F will be able to cross react with the native conformation of the pre-fusion spike on the surface of the virus. Accordingly, efforts to produce a vaccine against RSV have focused on developing vaccines that contain pre-fusion forms of RSV F protein (see, e.g., WO 2010/149745, WO 2010/149743, WO 2009/079796, and WO 2012/158613). However, these efforts have not yet yielded stable pre-fusion RSV F polypeptides that could be used as candidates for testing in humans.